Fatty acids are a principal metabolic fuel for liver, kidney and heart. Oxidation of fatty acyl-CoA to the corresponding trans-2,3-enoyl CoA is catalyzed by each of the acyl-CoA dehydrogenases. We propose to study the three-dimensional structure of the general acyl-CoA dehydrogenase from pig liver mitochondria. This enzyme has recently been under intensive investigation and progress has been made in elucidating the catalytic and structural properties of the enzyme. Detailed structural information obtained from high resolution X-ray structure analysis will enable us to relate the chemical functions to the structure of the protein and to define the catalytic mechanism. This enzyme has a molecular weight of 180,000 and contains four identical subunits, each containing one equivalent of flavin adenine dinucleotide. The enzyme has been crystallized in a form suitable for three-dimensional X-ray structure analysis. The crystals diffract to high resolution and the asymmetric unit contains two monomers of the tetrameric enzyme molecule. One good heavy atom contains and another potentially good derivative have been obtained. It is proposed to carry out the structural analysis to a resolution better than 4 angstroms initially and then extended to 3 angstroms analysis for elucidating the detailed mechanism of action of oxidation of acyl-CoA thioesters and transfer of elecrons to the electron transfer protein, the physiological oxidant of the dehydrogenase in the mitochondria.